In the search for energy producing fluids, such as oil and gas, it is often necessary to bore through extremely hard formations of the earth. Drill bits used in this industry typically have three roller cones or cutter cones designed to scrape and gouge the formation. A cutter cone having broad, flat milled teeth can very effectively scrape and gouge the formation. However, as the formation being drilled becomes more dense and hard, such milled teeth wear quickly with accompanying reduction in drilling efficiency. Even when coated with an abrasion-resistant material, milled teeth often crack or break when they encounter hard formations. Thus, milled teeth are typically unsuitable for boring through high density rock.
To alleviate this problem, engineers developed cone inserts that are formed from a hard, abrasion-resistant material such as sintered and compacted tungsten carbide. Typically, such inserts or compacts have a generally frustoconical or chisel-shaped cutting portion and are rugged and extremely hard and tough. These physical properties are necessary to break and pulverize hard formations. These generally shorter, more rounded, and extremely hard and tough inserts function to crush the formation, as opposed to scraping, cutting and gouging pieces from the formation.
These heretofore utilized rock bits with inserts improved the penetration rates, resistance of insert or tooth wear and breakage, and maximized tolerance to impact and unit loading. However, problems exist in providing inserts that are more easily manufactured, have hard, wear resistant elements that are more easily retainable with the body of the insert and which are not cost prohibitive and can be easily obtained.
Rotary cone drill bits are often used for drilling boreholes for the exploration and production of oil and gas. This type of bit typically employs three rolling cone cutters, also known as rotary cone cutters, rotatably mounted on spindles extending from support arms of the bit. The cutters are mounted on respective spindles that typically extend downwardly and inwardly with respect to the bit axis so that the conical sides of the cutters tend to roll on the bottom of a borehole and contact the formation.
For some applications, milled teeth are formed on the cutters to cut and gouge in those areas that engage the bottom and peripheral wall of the borehole during the drilling operation. The service life of milled teeth may be improved by the addition of tungsten carbide particles to hard metal deposits on selected wear areas of the milled teeth. This operation is sometimes referred to as "hardfacing." U.S. Pat. No. 4,262,761, issued Apr. 21, 1981 discloses the application of hardfacing to milled teeth and is incorporated by reference for all purposes within this application.
For other applications, sockets may be formed in the exterior of the cutters and hard metal inserts placed in the sockets to cut and gouge in those areas that engage the bottom and peripheral wall of the borehole during the drilling operation. The service life of such inserts and cutters may be improved by carburizing the exterior surface of the cutters. U.S. Pat. No. 4,679,640 issued on Jul. 14, 1987 discloses one procedure for carburizing cutters and is incorporated by reference for all purposes within this application.
A wide variety of hardfacing materials have been satisfactorily used on drill bits and other downhole tools. A frequently used hardfacing includes sintered tungsten carbide particles in an alloy steel matrix deposit. Other forms of tungsten carbide particles may include grains of monotungsten carbide, ditungsten carbide and/or macrocrystalline tungsten carbide. Satisfactory binders may include materials such as cobalt, iron, nickel, alloys of iron and other metallic alloys. For some applications loose hardfacing material is generally placed in a hollow tube or welding rod and applied to the substrate using conventional welding techniques. As a result of the welding process, a matrix deposit including both steel alloy melted from the substrate surface and steel alloy provided by the welding rod or hollow tube is formed with the hardfacing. Various alloys of cobalt, nickel and/or steel may be used as part of the binder for the matrix deposit. Other heavy metal carbides and nitrides, in addition to tungsten carbide, have been used to form hardfacing.
Both natural and synthetic diamonds have been used in downhole drill bits to provide cutting surfaces and wear-resistant surfaces. U.S. Pat. No. 4,140,189 teaches the use of diamond inserts protruding from the shirttail surface of a roller cone bit. Polycrystalline diamond (PCD) gauge inserts are frequently used on a wide variety of drill bits to prevent erosion and wear associated with harsh downhole drilling conditions. U.S. Pat. No. 4,140,189 is incorporated by reference for all purposes within this application.